The present invention relates generally to the field of liquid dispensing system and in particular to a system and method for efficiently charging a filter of a liquid dispensing system.
Many processes require accurate control over the amount and/or rate at which a fluid is dispensed by pumping apparatus. Both the rate and amount of processing fluid applied to, for example, a semiconductor wafer during fabrication of integrated circuits are very accurately controlled to ensure that the processing liquid is applied uniformly and to avoid waste and unnecessary consumption.
Fluid dispense systems in the prior art normally use positive displacement pumps to provide accurate metering of fluid. One type of positive displacement pump sometimes used in the prior art is a bellows-type pump, an example of which is disclosed in U.S. Pat. No. 4,483,665. In a typical bellows pump, fluid to be pumped enters a hollow tubular bellows through a one-way check valve. Usually, the discharge end of the bellows is constrained from movement, while the other end is connected to a reciprocating mechanical member that selectively works the bellows for longitudinal expansion and contraction. When contracted, fluid is expelled or pumped from the bellows under pressure. One problem with a bellows pump is that the pleats or convolutions in the bellows make it difficult to purge completely air or chemicals from the bellows. Air remaining in the bellows can create undesirable air bubbles.
A diaphragm-type positive displacement pump overcomes some of the problems associated with a bellows type of pump. A diaphragm pump has a diaphragm that divides a pumping chamber into two sections. A working fluid is pumped into and out of one section of the chamber to cause the diaphragm to move back and forth, thereby forcing process fluid to be drawn into and pushed out of the other half of the chamber. If the change in the volume of the working fluid within the chamber is accurately known, the volume of the process fluid within the chamber can also be known accurately, thus allowing for accurate metering. Diaphragm pumps are therefore often actuated by incompressible hydraulic fluid to achieve very accurate control over movement of the diaphragm. Examples of diaphragm pumps are disclosed in U.S. Pat. Nos. 4,950,134, 5,167,837, 5,490,765, 5,516,429, 5,527,161, 5,762,795, and 5,772,899.
Another type of well known positive displacement pump is a rolling membrane pump. A rolling membrane pump includes a reciprocating piston that displaces fluid within a pumping chamber. Unlike piston-type pumps that have a moving seal between the piston and the pumping chamber walls, a flexible membrane is attached to the piston and to the side walls of the chamber to prevent fluid from escaping between the walls and the piston. As the piston moves, the membrane rolls up and down the side of the pump.
These types of dispensing systems are being used, for example, in the manufacture of multi-chip modules (MCM), high-density interconnect (HDI) components and other semiconductor materials requiring the application of a thin layer of polyamide material as an inner layer dielectric. In addition to the unique mechanical and electrical properties that make polyamides ideally suited for use in the manufacture of semiconductors, polyamides also have physical properties that make it difficult to pump or supply the polyamides in exact amounts. Specifically, polyamides are viscous; most polyamides used in the manufacture of semiconductors have viscosities in excess of 400 poise. Fluids with viscosities this high are difficult to pump and difficult to filter. To be useful in a xe2x80x9ccleanroomxe2x80x9d environment the fluids must be filtered. Contamination in semiconductor device fabrication processes lowers yields and results in lost process fluid and production time. By its nature the filter becomes clogged during use. In positive displacement pumps, such as diaphragm-type positive displacement pumps, fluid flow rate through the filter decreases as the filter becomes more clogged, making it even more difficult to achieve repeatable, accurate dispense of fluid. Thus, the filter has to be changed periodically. One of the problems encountered in certain manufacturing processes such as those dispensing liquid photoresist chemicals is discontinuities formed in photoresist layers due to air bubbles that are introduced into the process. After the filter is changed, the system must be filled with fluid and air must be removed or purged, in order to prevent air bubbles from forming in the fluid. Positive displacement type pumps used to dispense fluid are also used to draw fluid into the system. Fully charging the system takes a long time, typically upwards of thirty minutes. This process causes a lot of down time during which the dispensing system cannot be used. However, since purging the air from the filter is an important step to avoid bubbles in the fabrication process, the purging process cannot be short circuited or eliminated.
The invention relates to an improved liquid dispensing system in which the time to purge air from the system and to charge a filter, especially after a filter change, is substantially reduced, thus reducing the down time of the system.
In a preferred embodiment of a liquid dispensing system employing the teachings of the present invention, a reservoir and filter are in line between a liquid source and a positive displacement pump, so that liquid flows from the source, into the reservoir and then into the filter. The filter is also vented through the reservoir. To charge the filter, a vacuum from a constant vacuum source such as Venturi is applied to the reservoir. This causes air to be drawn out of the filter and the reservoir, and liquid to be drawn into reservoir from a source. This results in the filter being purged of air and charged with liquid much faster than using only the positive displacement pump. Furthermore, venting the filter through the reservoir rather than the positive displacement pump reduces the opportunity for from the filter to enter the pump air from the filter. By having the filter at the input side of the pump and not in the dispense path, the accuracy and repeatability of the pump is also improved. Moreover, the dispense path and the suckback path of the positive displacement pump is direct and a filter valve is not necessary in the dispense path.
Other aspects and features of the invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.